1. Field of the Invention
The present invention relates to an improvement for a variable reluctance resolver for the detection of angular positions and rotational speeds.
2. Related Background Art
As a conventional variable reluctance resolver, there has been known a resolver in which the rotor core and stator magnetic poles are structured to change reluctance in the space between them in accordance with the positions of the rotor core thereby to enable the fundamental component of the reluctance variation to be N cycles per revolution of the rotor core. The structure is arranged to detect the rotational angular positions and rotational speeds by detecting the foregoing reluctance variations. Particularly, as a variable reluctance resolver for an a.c. servo motor, there is known a resolver structured with the stator magnetic pole having three phases and eighteen poles.
The above-mentioned conventional variable reluctance resolvers generate permeance between the rotor and stator corresponding to the facing relations between the teeth formed on both of them, and due to the spatial higher harmonic wave present in this permeance, the positional detection accuracy per tooth becomes degraded. This necessitates the provision of a correction circuit composed of ROM, D/A converter and other components for its signal processing circuit As a result, not only it is impossible to miniaturize the circuit, but also it is impossible to arrange any random combinations between the motor and the signal processing circuit because the correction data in the ROM are defined for each of the resolvers to determine the combination between them. These problems are yet to be solved.
When the stator magnetic poles are structured with three phases and eighteen poles, the permeance can be expressed in the following equation (1): EQU P=P.sub.0 +P.sub.1 cos.theta.+P.sub.2 cos2.theta.+P.sub.3 cos3.theta.+P.sub.4 cos4.theta.+P.sub.5 cos5.theta.. . . (1)
Here, P.sub.0 .noteq.P.sub.1 .noteq.P.sub.2 .noteq.P.sub.3 .noteq.P.sub.4 .noteq.P.sub.5.noteq.. . .
Also, the higher order components of more than P.sub.5 are so minute that their influence on the accuracy can be ignored. Therefore, the above-mentioned equation (1) can be simplified and expressed in the following equation (2): EQU P=P.sub.0 +.SIGMA.P.sub.i cosi.theta. (2)
Here, this is rearranged for the resolver signals fa (.theta.), fb (.theta.), and fc (.theta.) of the three phases A, B and C and expressed as follows: EQU fa(.theta.)=A.sub.0 +A.sub.1 cos.theta.+A.sub.2 cos2.theta.+A.sub.3 cos3.theta.+A.sub.4 cos4.theta. (3) EQU fb(.theta.)=A.sub.0 A.sub.1 cos(.theta.=120.degree.)+A.sub.2 cos2(.theta.-120.degree.)+A.sub.3 cos3(.theta.-120.degree.)+A.sub.4 cos4(.theta.-120.degree.) (4) EQU fc(.theta.)=A.sub.0 +A.sub.1 cos(.theta.+120.degree.)+A.sub.2 cos2(.theta.+120.degree.)+A.sub.3 cos3(.theta.+120.degree.)+A.sub.4 cos4(.theta.+120.degree.) (5
Then, if the phase conversion 3/2 is given to the above-mentioned equations (3) to (5), the signals to be inputted into the signal processing circuit are expressed in the following equations (6) and (7): EQU fc(.theta.)=3(A.sub.1 cos.theta.+A.sub.2 cos2.theta.+A.sub.4 cos4.theta.)/2 (6) EQU fs(.theta.)=3(A.sub.1 sin.theta.-A.sub.2 sin2.theta.+A.sub.4 sin4.theta.)/2 (7)
Therefore, in the signal processing circuit, the digital angle .phi., which will be expressed according the following equations (8) and (9), is calculated on the basis of the signals converted by the above-mentioned equations (6) and (7): EQU .theta.=.theta.+.DELTA..theta. (8) EQU .DELTA..theta.=tan.sup.-1 [(A.sub.2 -A.sub.4)sin3.theta./{A.sub.1 +(A.sub.2 +A.sub.4)cos3.theta.}] (9)
As is clear from these equations (8) and (9), an error .DELTA..theta. due to the spatial higher harmonic wave of the permeance is involved in the signal processing circuit. Consequently, a corrector is required for the signal processing circuit to correct the error .DELTA..theta. due to the spatial higher harmonic wave.
In order to solve this problem, three stator teeth are provided at intervals of 120.degree. each having three-phase alternating current excitation coil and output coil, respectively, as disclosed in Japanese Patent Laid-Open Application No. 1-218344, and a stator tooth having the same excitation coil and output coil in the reverse direction is provided at a position symmetrical to each of the phase stator teeth at an angle of 180.degree.. They are given as an A set of stator teeth. Thus, it is conceivable that the permeance components of second to fourth higher harmonic waves can be reduced by providing six B sets stator teeth having the same coils as the A set at positions each displaced by 90.degree. against the six set of the A set stator teeth. In this case, however, it is required to arrange two sets of excitation coil and output coil to input three-phase alternating current signals into the stator teeth individually; thus making it necessary to insulate the excitation coils and output coils. Accordingly, the structure becomes inevitably complicated such new that more time and labor are required for its assembly.